[POWERPC] PS3: Cleanup of os-area.c

Minor cleanup of the PS3 file os-area.c:
 o Correct file text header.
 o Add type names enum os_area_ldr_format, enum os_area_boot_flag,
   enum os_area_ctrl_button.
 o Change struct os_area_header.magic_num type to u8.
 o Add preprocessor macro SECONDS_FROM_1970_TO_2000.

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>